Dear forum members , I need you're help with one of the weirdest problem I've came a cross in a while.

My MT1500 mori seiki throws Alarm EX491 "spindle speed abnormal" when starting my main spindle on MDI/JOG/handle
after lets say 10 sec - and above 150 rpm.
But , and here its gets weird - when using it on a memory program who uses M35 (speed sync Mcode) it handles without throwing this alarm or any other for the duration of the M35 and spindle turning.
here is what i tried to solve it :
1) test encoder - manually turning and seeing if it acting wierd with the speeds (using the rpm scale on the controller)
2)There is proximity sensor who sits on the spindle parallels to the PG sensor (its definition by the book is Speed detection SQ390) - tested it and its functional and doesn't make any difference if it bypassed (its NO operated) .
3) checked the Spindle Amp with a lab - the didn't found anything bad in it .
4) I replaced the Magnetic Contactors that are connected to the main spindle - but still no progress .
5) look trough the Ladder with all the spindle Inputs from sensors and the outputs all lead to nothing major ( didn't got to checking functions blocks to be honest) .
all the ladder looking around made me think there is a process happening else where in PLC/IO on the background and I can't get to that area to see that clearly what throws me this alarm

sorry for the long post , but i realy could use you're help
and sure others like me !
with much appreciation shlomi